1. Field of the Invention
The present invention relates to a method of crystallizing silicon and a device fabricated using a method of crystallizing silicon, and more particularly, to a method of crystallizing amorphous silicon and a display device fabricated using a method of crystallizing amorphous silicon.
2. Discussion of the Related Art
Currently, demand for flat panel display devices that are thin, lightweight, and have low power consumption is increasing. In general, the flat panel display devices may be classified into two different categories based upon how light is produced. A first type of flat panel display devices include a light-emitting display device that emits light to display images, and a second type of flat panel display devices includes a light-receiving display device that uses external light sources to display images. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electro-luminescence display (ELD) devices are light-emitting displays, and liquid crystal display (LCD) devices are light-receiving displays.
The LCD devices are commonly used in laptop computers and as desktop computer monitors because of their superior resolution, color range, and image quality. The LCD devices make use of optical anisotropy and polarization properties of liquid crystal molecules to produce images. The liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes, and can be controlled by application of an electric field. Accordingly, the alignment of the liquid crystal molecules may be controlled by changing the applied electric field, wherein refraction of incident light is dependent upon the orientational alignment of the liquid crystal molecules. Thus, by properly controlling the applied electric field, a desired image can be produced.
Of the various types of LCD devices, active matrix liquid crystal display (AM-LCD) devices having thin film transistors (TFTs) and pixel electrodes arranged in a matrix configuration are commonly used because they can produce high quality images at reasonable costs. The TFTs commonly include polycrystalline silicon (p-Si) or amorphous silicon (a-Si) used as an active layer. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, it is commonly used for switching elements in the LCD devices. However, amorphous silicon (a-Si) is problematic when used in large-area LCD devices since amorphous silicon (a-Si) TFTs have relatively slow display response times, thereby limiting their suitability for the large-area LCD devices. In contrast, polycrystalline silicon TFTs provide much faster display response times and are well suited for use in the large-area LCD devices, such as laptop computers and wall-mounted televisions, which require greater field effect mobility. For example, the large-area LCD devices often require TFTs having field effect mobility greater than 30 cm2/Vs together with low leakage current.
During the formation of polycrystalline silicon for LCD device, laser treatment techniques are commonly used, wherein the polycrystalline silicon also can be used in the driving circuits for the TFT switching devices. Since polycrystalline silicon films are composed of crystal grains having grain boundaries, the larger the grains and the more regular the grain boundaries are the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, which would result in formation of a single crystal, would be valuable.
In a conventional process for forming a polycrystalline silicon layer, an intrinsic amorphous silicon layer is formed on an insulating substrate using a plasma chemical vapor deposition (PCVD) method or using a low pressure chemical vapor deposition (LPCVD) method. After the amorphous silicon layer is formed to have a thickness of about 500 Å (angstroms), it is re-crystallized into a polycrystalline silicon layer by using a crystallization method. The crystallization method is generally classified into one of an excimer laser crystallization (ELC) method, a solid phase crystallization (SPC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method.
In the ELC method, an insulating substrate having an amorphous silicon layer formed thereon is heated to a temperature of about 250° C. Then, an excimer laser beam is applied to the amorphous silicon layer to form a polycrystalline silicon layer. In the SPC method, the amorphous silicon layer is heat-treated at a high temperature for a long period of time to become crystallized into a polycrystalline silicon layer. In the MIC method, a metal layer is deposited on the amorphous silicon layer and the deposited metal is used for crystallization of the amorphous silicon layer, wherein large-sized glass substrates can be used as an insulating substrate for the amorphous silicon layer. In the MILC method, a metal is first formed on an amorphous silicon layer, and the amorphous silicon layer is crystallized into polycrystalline silicon by lateral growth of grains. In addition, an oxide pattern is formed on a predetermined active portion of the amorphous silicon layer.
The ELC method also has been used during annealing of amorphous silicon. The excimer laser allows areas of an amorphous silicon film to be exposed to very high temperatures for very short periods of time. Thus, annealing of the amorphous silicon is performed at an optimum temperature (less than 400 degrees Celsius) without degrading the underlying substrate upon which the silicon amorphous film is formed. However, the ELC method causes lack of control over some of the process steps, wherein an aperture size of the excimer laser is relatively small. Accordingly, multiple laser passes or shots may be required to complete an annealing process due to the reduced aperture size, power of the excimer laser, and thickness of the amorphous silicon film. Thus, the multiple shots of the excimer laser introduces non-uniformities during the annealing process. Furthermore, the substrates must be serially annealed rather than simultaneously annealed in a furnace, thereby increasing production costs of the TFTs made by the ELC method.
In the SPC method, a buffer layer is formed on a quartz substrate, which can withstand temperatures higher than 600° C., to prevent contamination from the quartz substrate. Then, an amorphous silicon layer is deposited on the buffer layer and heat-treated in a furnace at a high temperature to form a polycrystalline silicon layer. However, since the SPC method is performed at high temperatures for an extended period of time, it is difficult to acquire a desired crystalline silicon phase. For example, since the crystalline grains develop without a continuous directionality, the polycrystalline silicon layer may have an irregular surface. Accordingly, a thin film transistor that includes a gate insulating layer covering the polycrystalline silicon layer will result in the gate insulating layer having irregularities, thereby decreasing a breakdown voltage of the thin film transistor. In addition, the size of the polycrystalline silicon grains formed by the SPC method are very irregular, thereby deteriorating electrical characteristics of a device using the polycrystalline silicon layer. Furthermore, since the quartz substrate used for the SPC method is very expensive, fabrication costs are increased.
Unlike the SPC method, which uses an expensive quartz substrate, the MIC and MILC methods may utilize a relatively inexpensive glass substrate for forming the polycrystalline silicon. However, in the MIC and MILC methods, metal impurities may remain in the polycrystalline silicon, thereby deteriorating the quality of the polycrystalline silicon layer.
Another method of crystallizing amorphous silicon into polycrystalline silicon is a sequential lateral solidification (SLS) method. Crystallization using the SLS method make use of the tendency of silicon grains to grow laterally from interfaces between liquid and solid silicon, thereby producing grain boundaries that are perpendicular to the interfaces. During the SLS method, amorphous silicon is crystallized using a laser beam having a magnitude and relative motion that melts amorphous silicon to form laterally grown silicon grains upon re-crystallization. The SLS method is disclosed in detail by Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956-957, 1997.